1. Field of the Invention
This invention relates to paper covered gypsum board and its manufacture. More particularly, it relates to an improved gypsum board having increased resistance to fire in which there is incorporated mineral wool fibers.
As is well known, gypsum board comprising a cast gypsum core and cover sheets of paper or the like is widely used in building construction as, for example, gypsum wallboard, ceiling panels, lath and the like. In manufacturing such board, a water-stucco slurry fed from a mixer is spread over the face cover sheet and a back cover sheet applied thereover before the slurry has set. The set board is then cut to desired size and dried in a kiln. In use, the board is applied by means of clips, nails, screws or adhesives.
As the usage of such boards has expanded, specialty uses have placed a premium on certain properties. Specifically, fire ratings are required for numerous uses. When exposed to the heat and flames of a fire, the paper cover sheets first burn off the core. The cast gypsum core, predominantly calcium sulfate dihydrate, calcines with the gypsum giving up its chemically combined water (about 21% by weight of the gypsum). The water is slowly released as steam, effectively retarding heat transmission and disintegration of the board for a time as the gypsum is calcined. As the gypsum calcines it loses its inherent set gypsum strength. From studies of the actions of gypsum board when exposed to a fire, such as in a laboratory fire test, it has been generally evident that there is a substantial shrinkage of the board at sustained high temperature, with consequent cracking which not only contributes to passing excessive heat and hot gases through the test panel but also hastens the disintegration of the board under these adverse conditions.
2. Description of the Prior Art
To increase the fire resistant properties of gypsum products it has been conventional to introduce certain fibers and unexpanded vermiculite ore into the slurry of calcium sulfate hemihydrate (or calcined gypsum or gypsum stucco) and water during the board forming process. This concept is disclosed in U.S. Pat. Nos. 2,526,066; 2,681,863; 2,744,022; 2,803,575; 2,853,394; 3,454,456 and 3,616,173.
These patents basically teach the use of certain unexpanded vermiculite ores to offset the shrinkage of the board core during the heat exposure, the vermiculite ore expanding as the chemically combined water present in the gypsum is driven off. As this heating also tends to degrade the cohesiveness of the gypsum reducing the strength and integrity of the core, the special fiber component of the core formulation imparts a mechanical binding or matting effect to hold the calcining gypsum together and keep it from disintegrating and falling into the test furnace. Thus, U.S. Pat. No. 2,526,066 discloses increased resistance to the heat of a fire through the cooperative action of unexpected vermiculite and noncombustible fiber such as asbestos. U.S. Pat. No. 2,744,022 substitutes drawn textile glass fibers in the cooperating mixtures, and indicates that blown glass wool or mineral wool is unsatisfactory. The importance and the nature of the fiber portion of the cooperative mixtures is also stressed in U.S. Pat. No. 2,681,863, reporting that short strands of drawn textile fibers loosely bonded by a water soluble binder provide ready dispersibility of the fibers for individual distribution throughout the set gypsum core upon dissolution of the binder. This patent also indicates the unsuitability of blown glass wool or mineral wool for the fiber portion. In addition to the nature of the fiber, these patents also state that if such textile fibers are not present in recommended amounts, the core has a tendency to crack and break apart readily when exposed to severe heat. It also states that it is very difficult to incorporate into the core any amount even of the textile glass fibers which exceeds about 1.0 weight percent, because the fibers tend to mat together into clumps rather than dispersing uniformly in the aqueous slurry.
Further, U.S. Pat. No. 3,616,173 discloses that certain proportions of particular inorganic particles will further improve the fire resistant properties of board cores containing unexpanded vermiculite. Certain clays of less than 1 to about 40 micrometer (um) size and either colloidal silica or alumina of less than 1 micrometer size, or mixtures thereof, are alleged to provide some additional fire resistance. Also, U.S. Pat. No. 3,454,456 indicates that providing some proportion of the expanded perlite in the form of fine sized particles inhibits large surface fissuring and spalling on the board core. This patent calls for the use of an unexpanded vermiculite of a particle size which will pass through 50 U.S. Standard mesh sieve (297 um) and be retained upon a 140 mesh sieve (105 um) to accomplish low fire shrinkage and low spalling.